This invention relates to semiconductor lasers.
A conventional double heterostructure (DH) semiconductor laser typically includes a narrow bandgap active layer bounded by a pair of opposite-conductivity-type, wide bandgap, cladding layers. Thus, the interfaces between the active and cladding layers constitute a p-n heterojunction and a p-p or n-n (isotype) heterojunction. When the p-n heterojunction is forward biased, monority carriers are injected across it into the active layer. In the direction transverse to the layers, these carriers are confined to the active layer by the heterjunctions. The carriers undergo radiative recombination and generate stimulated emission in a direction parallel to the layers but perpendicular to a pair of mirrors, typically cleaved crystal facets. This type of laser is described by I. Hayashi in U.S. Pat. No. 3,758,875, assigned to the assignee hereof.
A somewhat more complex version of the semiconductor laser, known as the transverse junction stripe (TJS) laser, injects minority carriers across a p-n homojunction which is oriented transverse to the layers of an isotype DH configuration. As described in U.S. Pat. No. 4,183,038 granted to H. Namizaki et al, a TJS laser illustratively comprises an n-type isotype DH (i.e., an n-type or undoped active layer bounded by n-type cladding layers) and an elongated p-type zone which extends longitudinally along the resonator axis. This zone also extends transversely in one direction from the top surface toward the substrate, but transversely in the orthogonal direction only part way across the mirror faces. Typically, the p-type zone is formed by diffusing a p-type impurity (e.g., Zn) through a stripe mask on the top surface of the laser. Thus, the diffusion front defines a p-n homojunction which intersects the active layer at approximately a right angle and, when forward biased, injects minority carriers laterally from the p-type side of the active layer into the n-type side of the active layer.
This type of TJS laser suffers, however, from lack of good fabrication control because of the need to perform a relatively deep diffusion and from a lack of adequate waveguide control for the transverse position of the optical mode.